Correct off-by-one error in WP_TEST_FAIL

- Assert missed NULL terminator
- strncpy missed NULL terminator
Simplify list operation; correct indentation

In wp_free_wrapper:

- Remove redundant if(t != H && t->p != p) statement
- Simplify do { } while loop
- Remove extraneous if(t->next != NULL). t->next will never be NULL.
  These are famous last words

Additionally, correct switch/case indentation.
Wrap wp_tegen globals in struct.

Proofing this out:

- Move wp_tegen_initialized and wp_tegen_atexit into static struct
  wp_tegen called instance_data
- Create static wp_tegen * instance to instance_data
- Update references to wp_tegen_initialized to instance->initialized
- Update references to wp_tegen_atexit to instance->atexit